1. Field of the Invention
The present invention relates to an image processing method for rendering color images on a printer. More specifically, the present invention relates to a color separation method. Even more specifically, the present invention relates to a color separation method for a print process that, besides cyan, magenta, yellow and black colorants, includes additional colorants such as orange, green or blue colorants.
2. Description of the Related Art
Color printing systems generally rely on subtractive color mixing of a set of colorants to render different colors. A set of colorants that is widely used includes cyan, magenta, yellow and black colorants. A color separation process separates a color in a digital document into a set of colorant magnitudes that, when printed on top of each other, yield a faithful rendering of the color. A color separation process is usually based on inverting a mathematical printer model that expresses color as a function of printed colorant amounts. The coefficients of the function are usually obtained by printing and measuring a printer target of which the colorant values are specified.
The color separation for a cyan, magenta, yellow and black printing process involves a transformation from a three dimensional color space into a four dimensional colorant space. This additional degree of freedom is resolved by introducing additional constraints on the relationships between the magnitudes of the colorants. For example, a possible constraint could be that a given color should be rendered with the predetermined magnitude of black colorant in combination with magnitudes of cyan, magenta and yellow colorants. Setting constraints on the amount of black colorant is commonly referred to as a “grey component replacement” strategy.
The range of colors that a color print process is capable of rendering is called its color gamut. Practical constraints, such as the limited capability of a substrate to absorb large amounts of colorants in a given amount of time, may put limitations on the printable color gamut. Another limitation of the printable color gamut results from the broadband nature of absorption spectra of the cyan, magenta and yellow colorants. For example, a magenta dye not only absorbs light in the green portion of the visible spectrum, but to some extent also in the blue and red portions of the visible spectrum. When the magenta colorant is subtractively mixed with a cyan colorant to render a blue color, the blue color will look darker than expected because a considerable amount of the blue portion of the spectrum is absorbed by the magenta colorant. As a result, subtractive color rendering using commercial magenta and cyan colorants does not enable reproducing blue colors that are both bright and saturated. For similar reasons, subtractive color rendering using the available cyan, magenta and yellow colorants fails to render bright and saturated orange and green colors.
The printing industry addresses the above problems by using additional colorants that specifically target blue, orange and green portions of the visible spectrum. While this method is expensive, since it requires the use of additional printing stations to print these additional colorants, the solution is common in the packaging industry, where bright and saturated colors support the sale of a product.
The use of additional colorants greatly complicates the color to colorant separation process.
A first problem that arises is that no standardized targets exist for print systems that use more than four colorants. Additionally, such targets would create problems in printing, as most print processes break up when more than four colorants are printed on top of each other.
Another problem is that the introduction of additional colorants also introduces additional degrees of freedom that need to be managed in the color separation, i.e., the separation of color into five or more colorants is considerably more complex to manage than the separation of a color into four colorants.